There are many kinds of rail road cars for carrying particulate material, be it sand or gravel aggregate, plastic pellets, grains, ores, potash, coal or other granular materials. These materials are not liquid, yet may in some ways tend to flow in a quasi liquid-like manner under the influence of gravity. Many of those cars have an upper opening, or accessway of some kind, by which the particulate is loaded, and a lower opening, or accessway, or gate, by which the particulate material exits the car under the influence of gravity. The terminology “flow through” or “flow through rail road car” or “center flow” car, or the like, may sometimes be used for cars of this nature where lading is introduced at the top, and flows out at the bottom.
Some hopper cars have a primary construction that includes a pair of arcuate side walls and, typically, an arcuate roof, joined together in shell that has the general shape of a bulging inverted U. The sidewalls may be formed on a first radius or curvature about a first axis, and the roof sheet may be formed about a second radius of curvature about another axis. The roof and side wall sheets meet at the intersection of the two curves. There is, typically, a top chord structure located at or near this junction. Grain and plastic pellet cars often have this bulging arcuate shape.